An EL element composed of a fluorescent material layer prepared by coating a transparent electrode with a fluorescent powder of zinc sulfide activated by the addition of manganese and copper, the surface of which is coated with copper or copper sulfide, dispersed in an appropriate binder and drying has long been investigated by Vechtin England. Because of its high luminance and ease of production, such an EL element has attracted the attention of the industry from the standpoint of practical value. It is known that upon the appplication of a DC voltage in the initial condition after the preparation, this EL element allows the passage of current which first increases with time and then decreases with increasing luminance. Such a phenomenon is known as "forming phenomenon." After the forming phenomenon ends, the EL element begins to show stable emission characteristics. Despite its numerous advantages, this EL element as one practical problem that since the forming step generally needs a few hours to tens of hours and the initial stage of the forming step allows the passage of a large amount of current as 50 to 100 mA/cm.sup.2, a consideration must be given to how fast and efficiently the forming step is executed. It is known that the higher the temperature of the fluorescent material layer during the forming is, the faster progresses the forming. As pointed out by Alder and others, however, above 105.degree. C., an irreversible change takes place in the fluorescent material layer which has an adverse effect on the life of the element. Therefore, the temperature of the fluorescent material layer during the forming step cannot be too high.
In this forming step, the copper covering the fluorescent material in the transparent electrode is dispersed toward the counter electrode to form a high resistance layer of less copper concentration in the fluorescent layer in the extreme vicinity of the transparent electrode. Thus, this high resistance layer is a luminous layer. Accordingly, the key to the efficient forming is how efficiently the high resistance layer is formed.